Iron Availability Influences the Tolerance of Southern Ocean Phytoplankton to Warming and Elevated Irradiance

Michael J Ellwood,Philip W Boyd,Sarah M Andrew,Robert F Strzepek,Hugh T Morell

doi:10.3389/fmars.2019.00681

Abstract

The Southern Ocean is responsible for approximately 40% of oceanic carbon uptake through biological and physical processes. In the Southern Ocean, phytoplankton growth is limited by low iron (Fe) and light supply. Climate model projections for the Southern Ocean indicate that temperature, underwater irradiance and Fe supply are likely to change simultaneously in the future due to increasing anthropogenic carbon dioxide emissions. The individual effects of these environmental properties on phytoplankton physiology have been extensively researched, and culturing studies using Southern Ocean phytoplankton have shown that temperature and Fe will play a key role on setting growth under future conditions. To explore the potential responses of Southern Ocean phytoplankton to these environmental changes, we cultured the haptophyte Phaeocystis antarctica and the diatoms Chaetoceros flexuosus, Proboscia inermis, and Thalassiosira antarctica under two light and iron combinations and over a range of temperatures. Our study revealed that the thermal response curves of key Southern Ocean phytoplankton are diverse, with the highest growth rates measured at 5°C (the annual temperature range at the isolation sites is currently 1–4°C). Warming had species-specific effects on the photochemical efficiency of photosystem II (PSII; Fv/Fm), the functional absorption cross-section of PSII (σPSII), carbon:nitrogen ratio and cellular Chlorophyll a concentrations. Iron availability increased species’ ability to tolerate warmer conditions by increasing the upper limit for growth and subsequently increasing the thermal niche that each species inhabit.

Highlights

Phytoplankton productivity in the Southern Ocean plays an important role in the transfer of carbon from the atmosphere to the ocean’s interior, in a process called the biological carbon pump
The second section compares the responses of four species grown at present-day and future temperature conditions combined with variations in Fe and light availability
This section will focus on the impact of temperature on growth rates and changes in cell physiology coupled with measurements of photochemical physiology (Supplementary Table 3)

Summary

Introduction

Phytoplankton productivity in the Southern Ocean plays an important role in the transfer of carbon from the atmosphere to the ocean’s interior, in a process called the biological carbon pump. Southern Ocean productivity, in turn, is regulated by the availability of iron (Fe), light, and temperature, which influence the efficiency of the carbon pump (Sunda and Huntsman, 1997; Boyd et al, 2010). The effect of temperature on model diatom species has been well documented (Sunda and Huntsman, 2011); the interaction between temperature, light, and Fe on phytoplankton growth rate has only recently been explored in Southern Ocean species (Zhu et al, 2017; Boyd, 2019). Limited information is available on the thermal tolerance of specialized Southern Ocean phytoplankton, especially when temperature varies concurrently with other environmental variables (Boyd et al, 2013; Coello-Camba and Agustí, 2017)

Objectives

Methods

Results

Discussion

Conclusion